stream: add an assert() to an obscure seek case

This affects small seeks backwards while within the buffer. Demuxers
usually avoid this, so it's probably not triggered very often. (Although
demux_mkv.c potentially triggers it often, and it uses stream_peek() to
explicitly guarantee that it can use this code to seek back.) The
condition is complex enough to warrant an assertion.

stream/stream.c

@@ -563,6 +563,7 @@ bool stream_seek(stream_t *s, int64_t pos)
         int64_t x = pos - (s->pos - (int)s->buf_len);
         if (x >= 0) {
             s->buf_pos = x;
+            assert(s->buf_pos <= s->buf_len);
             return true;
         }
     }